Spark plugs can be used to initiate a combustion in internal combustion engines. Typically, spark plugs ignite a gas such as an air/fuel mixture, specifically in an engine cylinder or in a combustion chamber, in that a spark is produced transversely across a spark gap that is formed between two or more electrodes. The ignition of the gas by means of the spark triggers a combustion reaction in the engine cylinder, which is responsible for the power stroke of the engine. The high temperatures, the high electrical voltages, the rapid repetition of combustion reactions, and the presence of corrosive materials in the combustion gases can create a harsh environment in which the spark plug must function. The harsh environment can contribute to an erosion and to a corrosion of electrodes of spark plugs, which can negatively affect the performance of the spark plug over time. This in turn can potentially lead to misfires or other unwanted states.
To reduce erosion and corrosion of the electrodes of spark plugs, various kinds of precious metals and alloys have been used such as those that are composed of platinum and iridium. These materials are expensive, however. Consequently, the manufacturers of spark plugs try to minimize the quantity of precious metals used in an electrode. One approach to achieve this consists in using such materials only on an ignition tip or on a sparking section of the electrodes, i.e. in the place where a spark jumps across the spark gap.
Electrode devices for spark plugs typically include a central electrode device and a ground electrode device. The electrode devices preferably each have an electrode body composed of a first material that can be a metal or metal alloy such as a nickel alloy.
The electrode devices also preferably have an ignition tip that is at least partially made of a more precious metal material, for example iridium, platinum or ruthenium.
Various joining techniques are known for connecting the electrode body to the ignition tip, which can also be referred to as a precious metal part. Classically, the connection is produced by means of a welding method such as a laser welding method.
Document WO 2009/034318 A1 has disclosed a production method for an electrode with an iridium tip, which includes the step of joining the tip to a free end of the electrode by means of the friction welding method. The friction welding method is particularly carried out by means of a relative rotation between the tip and the electrode body while exerting a constant pressing force. The rotation is then ceased and the pressure is increased further until the relative rotation stops.
Document U.S. Pat. No. 9,705,292 B has disclosed a spark plug with a central electrode and a ground electrode. The ground electrode has an ignition tip mounted on it. The ignition tip has a discharge layer and a stress-relieving layer. The stress-relieving layer is embodied of a Pt/Ni alloy and is joined to the opposing surface by means of a diffusion layer. The discharge layer is composed of a Pt/Rh alloy and is joined to one side of the stress-relieving layer by means of a diffusion layer, specifically to the side opposite from the side with which the stress-relieving layer is joined to the ground electrode.
In the connection between a precious metal and an electrode body, it should be noted that the materials to be joined in this case have different melting points and as a rule also different thermal coefficients of expansion. Furthermore, the two materials must meet mechanical, chemical, and thermal life cycle requirements.
In addition to the above-described joining techniques, it is also possible to connect electrode bodies and precious metal parts to each other by means of electrical resistance welding or by means of electron beam welding. The precious metal part frequently has melting points above 2000° C. Nickel-based alloys for electrode bodies frequently have melting points in a range between 1350° C. and 1450° C.
Specifically in combinations of iridium and nickel-based materials, fractures can occur in the connection regions, which can lead to failures. A remedy for this can be in embodying the weld zone as a continuous contact surface. It is also possible to avoid fractures by means of particularly homogeneous alloys. The preparation of such homogeneous alloys, however, is very time-consuming.